Speech-to-text over x402 — Whisper large-v3 on an audio URL, FLAT price per clip (not per minute): up to 12 minutes and 25 MB for one payment. Word-level timestamps plus SRT/VTT subtitle output. No API key, no account. POST {audio_url, language, response_format}. Free: GET /v1/ai/transcribe/preview. Cryptographically signed (Ed25519) — every response is independently verifiable via /.well-known/obol-pubkey.